As the generation of devices advances, the goal of a higher performance trend cannot be achieved only by a scaling effect with a conventional bulk silicon wafer, and a new device structure is accordingly needed. An SOI wafer has been paid attention to as a starting raw material. The SOI wafer is manufactured by a bonding method (thinning a thickness by grinding and polishing), a SIMOX method, or a Smart Cut method (registered trademark: thinning a thickness by an ion implantation delamination). The SOI wafer has been mainly used which is manufactured by using the Smart Cut method (registered trademark), from the viewpoint of being capable of manufacturing an SOI layer in a wide thickness range in case of the fabrication of a thin SOI layer.
In the Smart Cut method, at least one gas ion of a hydrogen ion and a rare gas ion is implanted into a silicon single crystal bond wafer from a surface thereof to form an ion-implanted layer inside the wafer. Next, the ion-implanted surface of the bond wafer is brought into close contact with a surface of a base wafer directly or through a silicon oxide film. The wafers brought into close contact with one another are subjected to a delamination heat treatment to delaminate the bond wafer at the ion-implanted layer, and thereby a bonded wafer is manufactured. The bonded wafer is thereafter subjected to a bonding heat treatment to firmly bond the SOI layer delaminated from the bond wafer to the base wafer, and thereby the SOI wafer is obtained (See Patent Literature 1).
In this case, the temperature of the delamination heat treatment (usually 500° C. or more) is generally lower than that of the subsequent bonding heat treatment for enhancing bonding strength sufficiently. The bond wafer is delaminated due to the delamination heat treatment before sufficiently enhancing the bonding strength. After the delamination, the thickness of silicon is thin, and the physical resistance is therefore low. In case of low bonding strength, for example, the SOI layer flakes off and is easily damaged, and the defects thereof are thus generated.
In the event that, before the delamination is caused, the bonding strength can be enhanced to the extent that the flaking is not generated, however, sufficient bonding strength can be effected in the subsequent bonding heat treatment, and a high quality SOI wafer having few defects can be thereby manufactured.
Conventionally, for the purpose of improving the bonding strength and reducing the defects, Patent Literature 2 describes that a pre-annealing is performed at a temperature of 200 to 400° C. (for 10 to 360 minutes), and thereafter the delamination is performed by a heat treatment at a temperature of 500° C. Patent Literature 3 describes that the temperature is increased after introduction at 350° C. and a heat treatment is thereafter performed at a temperature of 500° C. for the delamination. Patent Literature 4 describes that the temperature is increased after introduction at 300° C. and heat treatment is thereafter performed at a temperature of 500° C. for the delamination.
However, even when the pre-annealing is performed before the delamination heat treatment as described above, an incidence of the SOI layer defects, such as blisters (portions where the SOI layer is not bonded to the base wafer) or voids (portions where the SOI layer is not formed), cannot be sufficiently reduced in some cases.